As a serious note though, on a micro enough scale, everything will become bumpy/not a single linear thing. On a macro enough scale it would be impossible to determine variation and appear flat.

In a common human sense, many things are flat and linear to normal human perception. Many crystal and metal formations have flat, straight, and sharp features.

And something theorized to be existing and straight would be (iirc) the planck length. Light so energized that the wave becomes essentially linear (please verify because this is memory only)